Known fuel injectors for use in diesel (compression ignition) engines include a valve needle which is movable within an injector body, often referred to as the nozzle body, to control fuel delivery through a plurality of injector outlets provided in the body. Movement of the valve needle may be controlled by several means, one of which involves the use of a control valve for controlling a control chamber pressure at a back end of the needle located remote from the injector outlet. The injector includes a delivery chamber which receives fuel from a high pressure fuel source, for example a common rail. Thrust surfaces of the valve needle are exposed to fuel pressure within the delivery chamber, whereas the back end of the valve needle is exposed to fuel pressure within the control chamber.
The control chamber receives fuel via a first flow path and the delivery chamber receives fuel through a second flow path, with both flow paths receiving fuel from the common rail. Typically, the common rail supplies fuel to a plurality of other injectors of the system also in a similar manner. A closing spring is typically provided to urge the valve needle closed in circumstances in which pressure in the rail is low e.g. engine start-up.
In order to move the valve needle within the nozzle body so as to open the outlets to commence injection, it is necessary to create a pressure differential across the valve needle so that the lifting force acting on the valve needle thrust surfaces is sufficient to overcome the closing force acting on the back end of the needle. The closing force acting on the needle is a combination of the spring force and the force due to fuel pressure within the control chamber. The control chamber is provided with a control valve operable to control whether the control chamber communicates with a low pressure reservoir or fuel drain.
In order to initiate injection, the control valve is actuated to open communication between the control chamber and the low pressure drain, thus allowing fuel pressure in the control chamber to decay. A point will be reached at which the downward force acting on the valve needle is reduced sufficiently for the lifting force to cause the needle to lift to open the outlets, thereby commencing injection.
In order to terminate injection, the control valve is de-actuated to close communication between the control chamber and the fuel drain and, hence, re-establishing high fuel pressure in the control chamber. A point will be reached at which the downward closing force acting on the valve needle is sufficient to overcome the upward lifting force acting on the valve needle and, hence, the valve needle is forced to close to terminate injection.
In order to benefit engine emissions it is desirable to have accurate control over the timing of valve needle movement, particularly at the end of injection. The actual time at which the valve needle is caused to close is determined by the rate at which fuel pressure is re-established within the control chamber following closure of the control valve. In known systems, it can be a disadvantage that recovery of control chamber pressure at the end of injection is too slow, or at least cannot be controlled with sufficient accuracy.
It is an object of the present invention to provide a fuel injection apparatus which addresses this problem.